birthdays manipulation

birthdays.rb

def reverse_birthdays birthdays
  result = {}
  birthdays.map { |day,names| names.map { |name| result[name] = day } }
  result
end
actual = {"2019-01-01"=>["karlo01", "karlo02"], "2019-01-02"=>["ben01", "ben02"]}
expected = {"karlo01"=>"2019-01-01", "karlo02"=>"2019-01-01", "ben01"=>"2019-01-02", "ben02"=>"2019-01-02"}
expected == reverse_birthdays(actual)

Hash#invert might also be interesting solution, but it doesn't work for this example:

actual.invert
=> {["karlo01", "karlo02"]=>"2019-01-01", ["ben01", "ben02"]=>"2019-01-02"}